1. Field of the Invention
The present invention relates to a method and a device for detecting the center of a wafer and, more particularly, it relates to a method and a device for detecting how the center of the wafer is shifted from the rotation center of a holder means on which the wafer is freely rotatably held.
2. Description of the Related Art
Various kinds of conventional semiconductor manufacturing apparatus including probing, etching and ashing ones carry out the aligning of wafers relative to the holder means. In the case of the apparatus for probing electric characteristics of chips on the wafer, rough pre-alignment of wafers is carried out by the prealignment device prior to close and accurate alignment of them.
This pre-alignment comprises aligning the center of the wafer with the rotation center of a wafer chuck on which the wafer is freely rotatably held, and positioning the orientation flat of the wafer formed by partly cutting away the wafer.
Japanese Patent Disclosure Hei 2-205049 discloses an apparatus capable of carrying out the pre-alignment optically without contacting the wafer. According to this apparatus, the semiconductor wafer is freely rotatably held on the wafer chuck. A light source is opposed to a light receiving element with the rim of the wafer interposed between them. The amount of light received by the light receiving element is measured while rotating the wafer by the wafer chuck.
When the wafer is shifted from the wafer chuck, the area at which light emitted from the light source is shielded by the wafer changes depending upon the wafer rotated. The amount of light received by the light receiving element is thus changed. Therefore, information relating to the contour of the wafer on the wafer chuck can be obtained from outputs applied from the light receiving element when it receives light. It can be thus detected how the wafer is shifted from the wafer chuck. This allows the wafer to be aligned with the wafer chuck.
In the case of this conventional pre-alignment apparatus, however, a specific linear light receiving element having an effective measuring length of several centimeters is needed. In addition, a light source having a light intensity which allows light uniform over several centimeters to be emitted is also needed. Further, it is needed that analog outputs of the light receiving element are converted into plural digital signals. Therefore, measuring results obtained are likely to change because of the light receiving element and the light source deteriorated as time goes by.
In addition to those opaque wafers which are made of silicon, quartz glass, crystal and gallium arsenic are recently used to make transparent wafers and these transparent wafers allow light to pass through them. When the above-mentioned conventional apparatuses are used relative to the wafers which allow light to pass through them. however, light emitted from the light source can pass through the wafers, thereby making it impossible to detect any change in the amount of light.